Should the <length> <length> offset be optional in a shadow?

1) [Minor:] the grammar contains two errors: it requires at least two shadows instead of one; and it makes the comma optional. The best fix depends on the answer to the next question.

2) The offset (<length> <length>) is optional and defaults to '0 0', which means that an omitted offset makes the shadow invisible and is thus almost never useful. There are at least two solutions: require an offset or make the default more useful.